Deformation of the modulation profile in phase holographic gratings.

Among all the parameters that characterize a phase grating, the most difficult to control is the modulation profile of the refractive index. In fact, it covers many scalar parameters that are the Fourier coefficients of the profile. To study the influence of processing baths on the modulation profile, in phase holographic gratings made of dichromated gelatin, we have observed that the shape of the profiles obtained sometimes presents a slight concavity or a convexity in the middle, leading to an increase or a decrease in the diffraction efficiency. We present the experimental results and a numerical study in the form of a theoretical prediction, which confirms the experimental observations.